Ask Mr. Smarty Plants

There are two plants in our Native Plant Database that have "Spanish Dagger" as one of their common names: Yucca aloifolia (aloe yucca) which is not native to Texas, and referred to as Spanish Bayonet  in this Floridata website; the other is Yucca treculeana (Don Quixote's lace) is shown as growing only in Texas. It probably doesn't much matter which you have, both are adaptable to the conditions around Austin and Leander.

Because we are not plant pathologists, we really don't know what is causing the problems your yucca is encountering. However, we found a website from the University of Florida Research Education Center, Yucca Production Guide, which gave very complete information on insect pests and diseases as well as instructions for care and propagation.  Generally speaking, yuccas are not considered to be seriously threatened by most of these problems. We aren't entomologists, either, but we did feel we should check out "icky yucca bugs". We learned that there really is a bug named the "yucca bug" (they omitted the "icky") and we found a site from the Bug Guide with pictures and some discussion of Halticoma valida. It has some pictures and you're right, it's one ugly bug. We're a little concerned about your spraying pyrethrins on the other plants. Is it possible that what you are seeing are the larvae of the yucca moth? The yucca and the yucca moth are co-dependant, in that the yucca moth is the sole pollinator of the yucca, and the yucca moth needs the seeds of the yucca for the larvae to feed on. 

In terms of saving the big centerpiece yucca, although they are ordinarily long-lived plants, this one may have reached the end of its life cycle. The spots may be from mealy bugs or one of the other less fearsome bugs discussed in the article from the Florida Research Education Center, and are likely not a serious threat. Browning lower leaves are a common sight on yucca, and the leaves can just be peeled off (very carefully). Don't try to overwater, this is a desert plant, and certainly don't water from above, as in sprinklers. One of the diseases mentioned in the article on Yucca was fungal in nature, and excess water, especially from above, contributes to that. The yucca is a very tough survivor plant; even if you cut it down, it will put new sprouts up from pieces of root. Keep fallen leaves and other trash cleaned up around the base and watch it for a while, it could bounce back, and if it doesn't, you have spares. 

Yucca treculeana

Yucca treculeana

Yucca treculeana

Yucca treculeana